Cadds

A Problem-Solving Process: One Story

Once upon a time, there was a bridge known as Galloping Gertie. This was the Tacoma Narrows Bridge, built to span the straits forming the entrance to Puget Sound in Washington state. Galloping Gertie started out the same way as many typical suspension bridges. It was constructed of high-quality materials, met every design criteria of the period, and was put together by a good construction crew. The bridge was completed and opened to traffic in the summer of 1940. However, it was not long before the bridge developed a distinctive swaying motion that grew more severe with time. This of course is not the typical behavior anyone would expect from a bridge, but it did happen. Films were made of Galloping Gertie swaying like a wet noodle. Eventually, the swaying became so severe that engineers closed the bridge on November 7th of the same year it opened. Not very long afterward, the bridge twisted, ripped itself apart, and collapsed into the narrows.

Now, suspension bridges are designed to incorporate a little swayback into them. This flexibility is key to the strength of any large structure. So the question then becomes: what happened with Galloping Gertie?

In the case of the Tacoma Narrows Bridge collapse, the answer came down to one factor not taken into account. The structural framing members were placed at precise equal distances from each other. This solution works in many structures, but in this instance, it proved a fatal error. The reason: every time the winds, their force amplified by the narrow channel, gathered within the framing, it subjected each portion of the bridge to the same torsional stresses. This set up what is known as a resonant frequency in the entire bridge structure. Equal force was transmitted throughout the whole framework and kept building up until it was strong enough to make the bridge sway in the wind. Ultimately, these forces became strong enough to violently tear the entire bridge down.

Finding engineering solutions to prevent things like a bridge twisting like a noodle until it crumbles is the subject of comprehensive analysis. Not only does this solve the mystery of a particular structural failure, it also prevents any such future occurrence. Such analysis is but one function of multidisciplinary groups employing whole teams of engineers from every field to engage in comprehensive problem-solving. To find out how one such firm operates, go to cadds.com.au for more details.